1. Field of the Invention
The present invention relates to pressure sensors and more particularly to dead-ended tube type high pressure sensors.
2. The Prior Art
Prior art systems for industrial pressure and temperature sensing involve independent wired sensors, which require significant installation time and cost by trained technicians. Pipework or pressure vessels must be penetrated twice which increases disturbance and doubles risk of leaks or failures, cf. a combined sensor.
Traditional designs for pressure sensors have utilised diaphragms and, especially at higher pressures, dead-ended tubes of constant circular cross section, where the hoop stress which arises and varies in the dead-ended tube due to internal pressure is exploited. Sensing techniques include foil, thick and thin film, and piezo resistive, strain gauges, and capacitive techniques using micro-machined semi-conductor materials.
It is also known in the art to use SAW devices to form wireless SAW based combined pressure and temperature sensors. Such SAW based systems have the advantages that they are passive, so that power and signal transmission are both provided by RF means from the interrogation unit without the need for a dc voltage source (battery) in the sensor itself. They also have good accuracy for pressure measurement—typically 1-3% of Full Scale, and also for temperature measurement—typically +/−2 C over range −40 C to +125 C. They have low hysteresis and drift, long life with low maintenance requirement, are rugged—ability to withstand vibration and rough handling, are non-degrading in target environment, are capable of operating in hazardous environments—intrinsically safe, can be installed by non-specialist staff, and are cost competitive.
Known wireless SAW pressure and temperature sensors typically utilise a hermetic package enclosing a quartz beam mounted on two supports upon the top surface of which three SAW resonators are located. The lid of the package constitutes an elastic diaphragm which presses on the center of the quartz beam. Variation in ambient pressure causes the diaphragm to deflect thereby bending the quartz beam and causing the central SAW resonator, located between the supports, to vary its frequency. The other two resonators, located outside the supports, enable temperature to be independently measured and the pressure signal to be temperature compensated.
Such prior art systems are ideally suited for pressure measurement within the range 0 to around 20 bar. However, much higher pressures, e.g. >100 bar, require a different approach.
A known system suitable for measurement of such higher pressures comprises a dead-ended, thin walled, tubular pressure sensor of uniform cross section. Such a sensor carries its own end load (due to pressure), and experiences elastic tensile hoop and axial stresses in the well known ratio 2:1 (hoop:axial). The equivalent hoop:axial strain ratio, for an elastic isotropic material, is dependent on Poisson's ratio (ν) and can be expressed (2−ν):(1−2ν). For steel ν=0.3, so the surface strain ratio is 1.7:0.4.